After the Mexican-American War, he performed frontier duties and escorted topographical parties, including a trip to California around Cape Horn in 1849. He commanded at Camp Picket during the Pig War on San Juan Island from August 10 to October 18, 1859.
Civil War
Casey was a Lieutenant Colonel of the 9th U.S. Infantry Regiment at the beginning of the war.[1]: 4
He was going to be promoted to Colonel of the 4th U.S. Infantry Regiment on October 9 of 1861,[1]: 186
however before the promotion could occur Casey was promoted to brigadier general of volunteers on August 31, 1861, shortly after arriving on the East Coast.[2][1]: 203 Casey eventually became a division commander in the IV Corps under Brig. Gen Erasmus Keyes. During the Battle of Seven Pines, Casey's division was attacked by D.H. Hill's Confederates and driven from the field in panic. Gen. George McClellan blamed them for the disaster, in spite of the fact that it was the smallest, least experienced, and least well-equipped division in the army and clearly should not have been placed in such a vulnerable location as the Seven Pines crossroads. Casey was removed from division command and replaced by Brig. Gen John J. Peck. For the remainder of the Peninsula Campaign, Casey and his former division were relegated to a post around army headquarters at Harrison's Landing and kept away from the front lines. After the Seven Days battles, when McClellan conducted a review of the army, the soldiers in Casey's division turned their backs and refused to cheer him.
He was promoted to Major General on May 31 of 1862.[3]: 466 He wrote the three-volume System of Infantry Tactics, including Infantry Tactics volumes I[4] and II,[5] published by the army on August 11, 1862, and Infantry Tactics for Colored Troops, published on March 9, 1863. The manuals were used by both sides during the Civil War. In December 1862 he was appointed to the board that ultimately convicted Maj. Gen. Fitz John Porter of disobedience and cowardice for his actions at the Second Battle of Bull Run.
At the end of the war, Casey received a brevet (honorary promotion) to the rank of major general dated March 15, 1865. He was mustered out of volunteer service and reverted to his regular army rank of colonel on August 24, 1865.
Postbellum activities and death
Casey retired from the army on July 8, 1868, at the age of 61, having served over 40 years on active duty.
In 1880 Casey became a veteran member of the Aztec Club of 1847—a military society originally composed of officers who had served in the occupation of Mexico City and later extended its membership to all United States officers who had served during the Mexican War and their descendants.[7][8]